Remove references to seL4_IPCBuffer.userData

This field is no longer needed to store a reference to the IPC buffer.

Get the bootinfo structure caputred by the runtime

Runtime now captures bootinfo via aux vectors.

Revert "seL4platsupport: Use strtol instead of sscanf to parse hex"

This reverts commit f61c0d582b3d2406317ff0276e8e7901105502b8.

The string being parsed is created using the %p format type to sprintf, as such it should
be parsed back with precisely the same format arguments.

seL4platsupport: Use strtol instead of sscanf to parse hex

libsel4platsupport: Construct environment variable for rootserver TCB cptr

Placing the TCB cptr into an environment variables allows for a generic way of talking
about the cptr of the initial thread in a process. As seL4 tells us by agreement where
our TCBs cptr is, we create the environment variable ourself to give to libc.

As the value of this cptr is currently an `enum`, which we cannot reference from
assembly, we also add a C function wrapper for retrieving it

Fix licenses

libsel4platsupport: Store bootinfo location as an environment variable

Modifies _sel4_start to store the bootinfo location given by the kernel in an environment
string. Implements a new platsupport_get_bootinfo to query the environment for this pointer.